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(#) AnimatedContent calls should use the provided `T` parameter in the content lambda

!!! ERROR: AnimatedContent calls should use the provided `T` parameter in the content lambda
   This is an error.

Id
:   `UnusedContentLambdaTargetStateParameter`
Summary
:   AnimatedContent calls should use the provided `T` parameter in the content lambda
Severity
:   Error
Category
:   Correctness
Platform
:   Any
Vendor
:   Jetpack Compose
Identifier
:   androidx.compose.animation
Feedback
:   https://issuetracker.google.com/issues/new?component=612128
Min
:   Lint 8.0 and 8.1
Compiled
:   Lint 8.7+
Artifact
:   [androidx.compose.animation:animation-android](androidx_compose_animation_animation-android.md.html)
Since
:   1.5.0
Affects
:   Kotlin and Java files and test sources
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/main/java/androidx/compose/animation/lint/AnimatedContentDetector.kt)
Tests
:   [Source Code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/test/java/androidx/compose/animation/lint/AnimatedContentDetectorTest.kt)
Copyright Year
:   2023

`content` lambda in AnimatedContent works as a lookup function that
returns the corresponding content based on the parameter (a state of
type `T`). It is important for this lambda to return content *specific*
to the input parameter, so that the different contents can be properly
animated. Not using the input parameter to the content lambda will
result in the same content for different input (i.e. target state) and
therefore an erroneous transition between the exact same content.`

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/foo/test.kt:14:Error: Target state parameter it is not used
[UnusedContentLambdaTargetStateParameter]
    ) { if (foo) { /**/ } else { /**/ } }
      -----------------------------------
src/foo/test.kt:18:Error: Target state parameter it is not used
[UnusedContentLambdaTargetStateParameter]
    content = { if (foo) { /**/ } else { /**/ } }
              -----------------------------------
src/foo/test.kt:23:Error: Target state parameter param is not used
[UnusedContentLambdaTargetStateParameter]
    ) { param -&gt; if (foo) { /**/ } else { /**/ } }
        -----
src/foo/test.kt:27:Error: Target state parameter param is not used
[UnusedContentLambdaTargetStateParameter]
    content = { param -&gt; if (foo) { /**/ } else { /**/ } }
                -----
src/foo/test.kt:32:Error: Target state parameter _ is not used
[UnusedContentLambdaTargetStateParameter]
    ) { _ -&gt; if (foo) { /**/ } else { /**/ } }
        -
src/foo/test.kt:36:Error: Target state parameter _ is not used
[UnusedContentLambdaTargetStateParameter]
    content = { _ -&gt; if (foo) { /**/ } else { /**/ } }
                -
src/foo/test.kt:40:Error: Target state parameter it is not used
[UnusedContentLambdaTargetStateParameter]
    ) { if (foo) { /**/ } else { /**/ } }
      -----------------------------------
src/foo/test.kt:43:Error: Target state parameter it is not used
[UnusedContentLambdaTargetStateParameter]
    content = { if (foo) { /**/ } else { /**/ } }
              -----------------------------------
src/foo/test.kt:47:Error: Target state parameter param is not used
[UnusedContentLambdaTargetStateParameter]
    ) { param -&gt; if (foo) { /**/ } else { /**/ } }
        -----
src/foo/test.kt:50:Error: Target state parameter param is not used
[UnusedContentLambdaTargetStateParameter]
    content = { param -&gt; if (foo) { /**/ } else { /**/ } }
                -----
src/foo/test.kt:54:Error: Target state parameter _ is not used
[UnusedContentLambdaTargetStateParameter]
    ) { _ -&gt; if (foo) { /**/ } else { /**/ } }
        -
src/foo/test.kt:57:Error: Target state parameter _ is not used
[UnusedContentLambdaTargetStateParameter]
    content = { _ -&gt; if (foo) { /**/ } else { /**/ } }
                -
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/foo/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package foo

import androidx.compose.animation.*
import androidx.compose.runtime.*

val foo = false

@Composable
fun Test() {
    AnimatedContent(
        foo,
        contentKey = { if (foo) { /**/ } else { /**/ } }
    ) { if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { if (foo) { /**/ } else { /**/ } },
        content = { if (foo) { /**/ } else { /**/ } }
    )
    AnimatedContent(
        foo,
        contentKey = { param -> if (foo) { /**/ } else { /**/ } }
    ) { param -> if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { param -> if (foo) { /**/ } else { /**/ } },
        content = { param -> if (foo) { /**/ } else { /**/ } }
    )
    AnimatedContent(
        foo,
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } }
    ) { _ -> if (foo) { /**/ } else { /**/ } }
    AnimatedContent(
        foo,
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } },
        content = { _ -> if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { if (foo) { /**/ } else { /**/ } }
    ) { if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { if (foo) { /**/ } else { /**/ } },
        content = { if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { param -> if (foo) { /**/ } else { /**/ } }
    ) { param -> if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { param -> if (foo) { /**/ } else { /**/ } },
        content = { param -> if (foo) { /**/ } else { /**/ } }
    )
    Transition(foo).AnimatedContent(
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } }
    ) { _ -> if (foo) { /**/ } else { /**/ } }
    Transition(foo).AnimatedContent(
        contentKey = { _ -> if (foo) { /**/ } else { /**/ } },
        content = { _ -> if (foo) { /**/ } else { /**/ } }
    )
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:/compose/animation/animation-lint/src/test/java/androidx/compose/animation/lint/AnimatedContentDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `AnimatedContentDetector.unreferencedParameters`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=612128.

(##) Including

!!!
   This is not a built-in check. To include it, add the below dependency
   to your project.

```
// build.gradle.kts
implementation("androidx.compose.animation:animation-android:1.8.0-alpha07")

// build.gradle
implementation 'androidx.compose.animation:animation-android:1.8.0-alpha07'

// build.gradle.kts with version catalogs:
implementation(libs.animation.android)

# libs.versions.toml
[versions]
animation-android = "1.8.0-alpha07"
[libraries]
# For clarity and text wrapping purposes the following declaration is
# shown split up across lines, but in TOML it needs to be on a single
# line (see https://github.com/toml-lang/toml/issues/516) so adjust
# when pasting into libs.versions.toml:
animation-android = {
    module = "androidx.compose.animation:animation-android",
    version.ref = "animation-android"
}
```

1.8.0-alpha07 is the version this documentation was generated from;
there may be newer versions available.

NOTE: These lint checks are **also** made available separate from the main library.
You can also use `androidx.compose.animation:animation-lint:1.8.0-alpha07`.


[Additional details about androidx.compose.animation:animation-android](androidx_compose_animation_animation-android.md.html).
(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("UnusedContentLambdaTargetStateParameter")
  fun method() {
     AnimatedContent(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("UnusedContentLambdaTargetStateParameter")
  void method() {
     AnimatedContent(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection UnusedContentLambdaTargetStateParameter
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="UnusedContentLambdaTargetStateParameter" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'UnusedContentLambdaTargetStateParameter'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore UnusedContentLambdaTargetStateParameter ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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